The hydrogeologic setting was described and preliminary estimates of hydrologic components prepared for the Bull Run Lake and for the Bull Run Lake drainage basin, in the Cascade Range of northwestern Oregon. The 0.73-square-mile lake and the 3.44-square-mile drainage basin lie within the Bull Run Watershed, the principal water supply for the Portland, Oregon, metropolitan area. During periods of high demand or low inflows to the watershed, the City of Portland, Bureau of Water Works, releases water from Bull Run Lake to augment the supply.
Bull Run Lake is impounded by a natural dam formed by a landslide. Outflow of ground water from the lake through the landslide emerges as springflow at the toe of the landslide and forms the headwaters of the Bull Run River. The approximately 4,300-Mgal (million gallons) discharge of the Bull Run River measured below the springs during the 1993 water year is composed of (1) outflow of ground water from Bull Run Lake through the landslide (approximately 60 percent), (2) ground water originating from the contributing drainage area between the lake and the springs (approximately 34 percent), (3) streamflow from Bull Run Lake (approximately 5 percent), and (4) surface runoff (streamflow and overland flow) from the contributing drainage area between the lake and the springs (approximately 1 percent). Estimated ranges for inflows to the Bull Run Lake drainage basin during the 1993 water year were about 3,400 to 9,200 Mgal from precipitation from rain and snow, and about 0 to 3,300 Mgal from fog drip.
Estimated ranges for outflows from the lake basin, listed from largest to smallest, were about 1,800 to 3,400 Mgal for ground-water outflow through the landslide; about 600 to 1,800 Mgal for evapotranspiration from the land surface; about 170 to 410 Mgal for lake evaporation; and about 0 to 400 Mgal for streamflow from the lake. Ground- water outflow through the consolidated rocks could not be evaluated owing to the lack of data. The lake storage increased by a range of from about 1,700 to 1,900 Mgal. Changes in ground-water storage and soil-moisture storage could not be evaluated as a result of insufficient data.
Estimated inflows to Bull Run Lake from precipitation on the lake surface during the 1993 water year ranged from about 600 to 1,600 Mgal. Inflows from ground water and surface runoff could not be evaluated owing to the lack of data. Estimated ranges for outflows from the lake were about 1,800 to 3,400 Mgal from ground-wateroutflow through the landslide, about 170 to 410 Mgal from lake evaporation, and about 0 to 400 Mgal from streamflow. Outflow of ground water through the consolidated rocks could not be evaluated owing to the lack of data. Lake storage increased by a range of from about 1,700 to 1,900 Mgal.
Suggestions for further study include (1) evaluation of the surface-runoff component of inflow to the lake; (2) use of a cross-sectional ground-water flow model to estimate ground-water inflow, outflow, and storage; (3) additional data collection to reduce the uncertainties of the hydrologic components that have large relative uncertainties; and (4) determination of long-term trends for a wide range of climatic and hydrologic conditions.
|Title||Hydrogeologic setting and preliminary estimates of hydrologic components for Bull Run Lake and the Bull Run Lake drainage basin, Multnomah and Clackamas counties, Oregon|
|Authors||Daniel T. Snyder, Dorie L. Brownell|
|Publication Subtype||USGS Numbered Series|
|Series Title||Water-Resources Investigations Report|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Oregon Water Science Center|